1. Field of the Invention
The present invention generally relates to measurement systems, and more particularly to a system for taking displacement measurements of an object having a regular pattern preprinted thereon.
2. Discussion of the Related Art
There are a wide variety of known systems in which detecting and/or measuring the position or displacement of an object is important. For example, the system disclosed in U.S. Pat. No. 5,578,813, assigned to the assignee of the present invention and also co-invented by the inventor of the present invention, is a system and method for determining relative movement between a hand-held scanner and a web of material (i.e., piece of paper). Specifically, that system utilized an illumination/imaging sensor to detect relative movement between the scanner and the web of material by identifying structure-related properties of the web of material. The inherent structure-related properties of the web (such as paper fibers, or other constituents) were used for navigational purposes, namely to identify the navigational path of the scanner, so that the image scanned could be reconstructed electronically.
Another system is disclosed in U.S. Pat. No. 5,291,131, which discloses an apparatus for measuring the elongation of a circulating chain (elongation resulting from component wear, stretching, or otherwise). The system disclosed therein uses two sensors (e.g., magnetic or optical) disposed a predetermined distance apart along the path of the circulating chain. The distance between the two indices is calculated on the basis of the calculated speed of the moving body and the time which elapses from when a first index passed a first sensor until a second index passed a second sensor. By continuing this observation over time, and comparing the calculated distances, chain elongation can be measured. Further, similar systems predating the '131 patent include those disclosed in U.S. Pat. Nos. 4,198,758 and 4,274,783, both entitled "Chain Measuring and Conveyor Control System", co-invented by the inventor of the present invention.
As is also known in the prior art, the print head of some ink jet printers is configured to move across the print width of a sheet of paper. The deposition of ink from the print head to the sheet of paper is closely controlled based in part on positional information of the print head. In some such printers, this positional information is obtained by stretching a transparent sheet of material across the span covered by the width of the sheet of paper. This sheet of material is passed through a slot in the print head, and contains periodically-spaced demarcation lines. An optical emitter/detector pair is disposed across the slot, and is configured to count the demarcation lines. By maintaining an accurate count of the demarcation lines, the system can maintain information related to the position of the print head along the sheet of paper. In such a system, the sheet of material is held stationary, while the emitter/detector pair (which is fixed to the print head) moves in relation to the sheet of material.
In systems like those mentioned above, as well as many other systems, there is a desire to obtain accurate and precise displacement measurements of an object. Accordingly, it is desired to provide an apparatus and method that effectively measures object displacements.